1. Field of the Invention
The present invention relates to an aberration compensating optical element having a diffractive structure, an optical system comprising this aberration compensating optical element, an optical pickup device, a recorder and a reproducer. Further, the present invention relates to an aberration compensating optical element which is disposed on an optical path between a light source and a high NA objective lens having at least one plastic lens and which can minimize a change in a spherical aberration of the objective lens, which is caused by a temperature change; an optical system comprising this aberration compensating optical element and a high NA objective lens having at least one plastic lens, wherein the optical system is used for an optical pickup device for carrying out at least one of the record of information on an optical information recording medium and the reproduction of information from an optical information recording medium; an optical pickup device comprising this optical system, for carrying out at least one of the record of information on an optical information recording medium and the reproduction of information from an optical information recording medium; a recorder comprising this optical pickup device, for carrying out the record of at least one of a sound and an image on an optical information recording medium; and a reproducer for carrying out the reproduction of at least one of a sound and an image from an optical information recording medium.
2. Description of Related Art
In recent years, a new high density storage optical pickup system using a blue-violet semiconductor laser light source having an oscillation wavelength xcex of about 400 nm and an objective lens having an enhanced numerical aperture (NA) of about 0.85, has been researched and developed. On an optical disk having a diameter of 12 cm, which is the same as that of a DVD (NA=0.6, xcex=650 nm, storage capacity 4.7 GB), for example, an optical pickup system (NA=0.85, xcex=400 nm) can record 25 GB of information.
However, when such a high NA objective lens and a short wavelength light source having the oscillation length of about 400 nm are used, a problem that an axial chromatic aberration occurs at an objective lens, is caused. In general, a laser light emitted from a semiconductor laser has a single wavelength (single mode), and it is thought that the axial chromatic aberration does not occur. In practice, there is some possibility that the mode hopping in which a center wavelength of a laser light is instantly hopped about several nanometers by changing the temperature, the output of the light or the like, occurs. Because the mode hopping is a phenomenon that a wavelength is changed so instantly that the focusing of an objective lens cannot follow the wavelength change, if a chromatic aberration occurring due to the objective lens is not compensated, a defocus component caused by the mode hopping is added. As a result, a wavefront aberration increases. In case of using a high NA objective lens or a short wavelength light source, the wavefront aberration caused at the mode hopping specially increases for the following reason. When due to a wavelength change xcex94xcex, the spherical aberration is not changed at the objective lens and the back focus fb is changed by xcex94fb, if the objective lens is focused in an optical axis direction against a change of the back focus, the root mean square value Wrms of the wavefront aberration is 0. When the objective lens is not focused, the value Wrms is expressed by the following Formula (1).
Wrms=0.145xc2x7{(NA)2/xcex}/|xcex94fb|xe2x80x83xe2x80x83(1) 
For example, when an optical pickup system for DVD (NA=0.6, xcex=650 nm) is compared with one for an optical disk (NA=0.85, xcex=400 nm), in case of the same xcex94fb, the wavefront aberration occurring in the latter optical pickup system increases by 3.26 times. That is, if the permissible value of the wavefront aberration is the same in both systems, the permissible value of |xcex94fb| decreases by 1/3.26 in the latter system. Therefore, it is required that the axial chromatic aberration on a wave surface of a light which transmits through the objective lens and is condensed on a storage surface of an optical disk, should be small.
In such a high density optical pickup system, in order to save the cost thereof and to lighten it, it is desired that an objective lens is a plastic lens like a conventional CD system and a DVD system. However, in a high NA plastic objective lens, when the temperature changes, the change in the spherical aberration, which is caused by the refractive index change of plastic, is large because it increases in proportion to the fourth power of NA. Therefore, in practical use, the above change becomes a significant problem.
As a cemented doublet type of objective lens for optical disk, which is formed so as to compensate the chromatic aberration, ones disclosed in Japanese Patent Publications (laid-open) No. Tokukai-Sho 61-3110 and No. Tokukai-Sho 62-286009, are known. The above lens which is formed by combining a lens made of low dispersion material having a positive refractive power and a lens made of high dispersion material having a negative refractive power, is unsuitable for an objective lens used for a optical disk, which is required to be light. Because there is a limit of the dispersion of the material, the above lens itself becomes heavy in order to obtain a high NA and to increase the refractive power of each lens.
In Japanese Patent Publication (laid-open) No. Tokukai-Hei 11-174318, an objective lens having a doublet lens structure, wherein the numerical aperture on an optical disk side is 0.85, and a hologram is provided on an optical surface in order to compensate the axial chromatic aberration, is disclosed. However, when the hologram comprises a ring-shaped zone structure having a plurality of fine steps which are formed into concentric circles, in a high objective lens which has a tendency to decrease a curvature of the optical surface, a shadow of the ring-shaped zone structure largely influences the transmitted light and then the transmittance of the light decreases. Therefore, the above objective lens is unsuitable for a high density storage optical pickup system as an information writing system which is required to have a high light utilization efficiency.
As an aberration compensating optical element for compensating the axial chromatic aberration of the objective lens, one disclosed in Japanese Patent Publication (laid-open) No. Tokukai-Hei 6-82725, is known. When the aberration compensating optical element in which a plurality of steps are formed as ring-shaped zones having the form of concentric circles around an optical axis on a plane surface perpendicular to the optical axis, is disposed in parallel light flux, a reflected light in a diffractive structure returns in the same direction as an incident light. As a result, a ghost signal is generated in a detection system of the optical pickup device. Therefore, The above lens is unsuitable for an aberration compensating optical element used in an optical pickup system.
In order to solve the above problem, an object of the present invention is to provide an optical system for optical pickup device, which can compensate the axial chromatic aberration with a relatively simple structure even though a light source having a bad monochromaticity or a light source in which a wavelength of the light suddenly changes, is used in a high density storage optical pickup system or the like, and which can be manufactured in a low cost; an optical pickup device comprising the above optical system; and a recorder and a reproducer, comprising the above optical pickup device.
Another object is to provide an aberration compensating optical element which can compensate the spherical aberration and the sine condition because of a large numerical aperture on the optical information recording medium side, and can compensate the axial chromatic aberration when an objective lens in which the axial chromatic aberration remains, is used in order to downsize an optical device, to thin it, to lighten it, and to decrease the cost thereof.
Further, another object is to provide an optical system used in optical pickup device, for suppressing the change in the spherical aberration, which is caused at an objective lens by the temperature change, with a relatively simple structure, even though a high NA plastic objective lens is used in, for example, a high density storage optical pickup system; an optical pickup device comprising the above optical system; and a recorder and a reproducer, comprising the above optical pickup device.
Further, another object is to provide an aberration compensating optical element which can compensate the change in the spherical aberration, which is caused at an objective lens by the temperature change, when a plastic objective lens in which the change of the spherical aberration is large because of the temperature change, is used.
In order to accomplish the above-mentioned object, in accordance with the first aspect of the present invention, an aberration compensating optical element comprises:
a diffractive structure having a plurality of ring-shaped zone steps formed on at least one surface of the aberration compensating optical element;
wherein the aberration compensating optical element is adapted for being disposed on an optical path between a light source for emitting a light having a wavelength of not more than 550 nm, and an objective lens made of a material having an Abbe constant of not more than 95.0 at a d-line; and
wherein the following formula (2) is satisfied:
Pxcex1 less than Pxcex0 less than Pxcex2xe2x80x83xe2x80x83(2), 
where Pxcex0 is a paraxial power (mmxe2x88x921) of the aberration compensating optical element at the wavelength xcex0 of the light emitted from the light source;
Pxcex1 is a paraxial power (mmxe2x88x921) of the aberration compensating optical element at a wavelength xcex1 which is 10 nm shorter than the wavelength xcex0; and
Pxcex2 is a paraxial power (mmxe2x88x921) of the aberration compensating optical element at a wavelength xcex2 which is 10 nm longer than the wavelength xcex0.
The above formula (2) is a condition of the paraxial power of the aberration compensating optical element disposed between this light source emitting a light having a wavelength of not more than 550 nm and the objective lens in which the axial chromatic aberration remains, to compensate the axial chromatic aberration occurring at the objective lens, in an optical pickup device in which the light source having a bad monochromaticity or a light source in which a wavelength of the light suddenly changes, is used. The formula (2) has a meaning that the axial chromatic aberration caused in the whole system of the optical pickup system including the aberration compensating optical element and the objective lens is compensated by increasing the paraxial power of the aberration compensating optical element at the wavelength xcex2 which is 10 nm longer than the wavelength xcex0 of the light emitted from the light source, in order to over-compensate the axial chromatic aberration caused at the aberration compensating optical element, and by canceling the under-corrected axial chromatic aberration caused by the objective lens at the wavelength xcex2. By combining the aberration compensating optical element according to the present invention and the objective lens, even though the axial chromatic aberration caused at the objective lens is not precisely compensated, it is possible to use the objective lens causing a problem relating to the remaining axial chromatic aberration thereof, in an optical pickup device in which a short wavelength light source having a bad monochromaticity is provided. The paraxial power of the aberration compensating optical element is the power of the whole system of the aberration compensating optical element, which is calculated by combining the refractive power as a refractive lens and the diffractive power generated by only a diffractive structure.
In accordance with the second aspect of the present invention, an aberration compensating optical element comprises:
a diffractive structure having a plurality of ring-shaped zone steps formed on at least one surface of the aberration compensating optical element;
wherein the aberration compensating optical element is adapted for being disposed on an optical path between a light source for emitting a light having a wavelength of not more than 550 nm, and an objective lens made of a material having an Abbe constant of not more than 95.0 at a d-line; and
wherein at least one ring-shaped zone step having a step distance xcex94 (mm) in a direction of an optical axis between adjacent steps of the plurality of ring-shaped zone steps is formed within an effective diameter so that m, defined by following equations:
m=INT(Y), 
Y=xcex94xc3x97(nxe2x88x921)/(xcex0xc3x9710xe2x88x923)xe2x80x83xe2x80x83(3), 
xe2x80x83is an integer except 0 and xc2x11,
where INT(Y) is an integer obtained by rounding Y, xcex0 is the wavelength (nm) of the light emitted from the light source, and n is a refractive index of the aberration compensating optical element at the wavelength xcex0 (nm).
The above formula (3) has a meaning that the difference xcex94 in a direction of an optical axis between adjacent steps of the plurality of ring-shaped zone steps of the aberration compensating optical element is determined so that a diffracted light amount of the higher order diffracted light having a diffraction order which is two or more, is larger than those of the diffracted lights having the other diffraction orders, when the incident light flux is diffracted by the diffractive structure formed on the optical surface of the aberration compensating optical element.
In accordance with the third aspect of the present invention, an aberration compensating optical element comprises:
a diffractive structure having a plurality of ring-shaped zone steps formed on at least two surfaces of the aberration compensating optical element;
wherein the aberration compensating optical element is adapted for being disposed on an optical path between a light source for emitting a light having a wavelength of not more than 550 nm, and an objective lens made of a material having an Abbe constant of not more than 95.0 at a d-line.
In a general optical material, when the wavelength of the light becomes short, the change in the refractive index, which is caused by a slight wavelength change, becomes large. In case of using a short wavelength light source emitting a light having a wavelength xcex of not more than 550 nm, when a slight wavelength change is caused, the axial chromatic aberration caused at the objective lens becomes large. Therefore, the required power of the aberration compensating optical element as a diffractive lens must be large in order to compensate the axial chromatic aberration caused at the objective lens. When the power of the objective lens is xcfx86OBJ, and the power of the aberration compensating optical element is xcfx86SA, in order not to change the back focus by the wavelength change in the compound system including the objective lens and the aberration compensating objective element, the following formula (4) may be satisfied:
dxcfx86SA/dxcex=xe2x88x92dxcfx86OBJ/dxcexxe2x80x83xe2x80x83(4). 
On the other hand, the relation between the change in the power of the objective lens, which is caused by the wavelength change, and the change in the back focus, is expressed by the following formula (5). Because the power of the aberration compensating optical element as a diffractive lens is proportional to the wavelength, the power is expressed by the following formula (6).
dxcfx86OBJ/dxcex=xe2x88x92(dfB/dxcex)xc2x7dxcfx86OBJ2xe2x80x83xe2x80x83(5) 
dxcfx86SA/dxcex=xcfx86SA/xcexxe2x80x83xe2x80x83(6) 
When the formulas (5) and (6) are substituted for the formula (4), the power xcfx86SA of the aberration compensating optical element as a diffractive lens is expressed by the following formula (7):
xcfx86SA=(dfB/dxcex)xc2x7dxcfx86OBJ2xe2x80x83xe2x80x83(7). 
For example, in case of a general objective lens for DVD, which has a focal length of 3.33 mm, a working wavelength of 650 nm, an NA of 0.6, an entrance pupil with a diameter xcfx86 of 4 mm, and a xcexdd of 55, because dfB/dxcex is 0.15 xcexcm/nm, the power xcfx86SA of the aberration compensating optical element is determined as shown in the following formula (8):
xcfx86SA=0.15xc3x9710xe2x88x923xc2x7650xc2x7(1/3.33)2=1/92.3 (mmxe2x88x921)=1.1xc3x9710xe2x88x922 (mmxe2x88x921)xe2x80x83xe2x80x83(8). 
In case of the objective lens for a high density storage optical pickup device, which has a focal length of 2.35 mm, a working wavelength of 405 mm, an NA of 0.85, an entrance pupil with a diameter xcfx86 of 4 mm, and a xcexdd of 55, because dfB/dxcex is 0.40 xcexcm/nm, the power xcfx86SA of the aberration compensating optical element is determined as shown in the following formula (9):
xcfx86SA=0.40xc3x9710xe2x88x923xc2x7405xc2x7(1/2.35)2=1/34.1 (mmxe2x88x921)=2.9xc3x9710xe2x88x922 (mmxe2x88x921)xe2x80x83xe2x80x83(9). 
That is, it is required that the aberration compensating optical element for a high density storage optical pickup device has a power which is 2.7 times larger than one for DVD. In practical, because an objective lens for a high density storage optical pickup device has a large NA, a focal depth is small. Therefore, it is required that the axial chromatic aberration is precisely compensated. The required power of the aberration compensating optical element for a high density storage optical pickup device becomes larger than the formula (9).
The optical path difference "PHgr" generated in a transmitted wave surface by the aberration compensating optical element having the power xcfx86SA is expressed by the following formula (10) as a function of the height h from the optical axis,
"PHgr"=(xcfx86SA/2)xc2x7h2xe2x80x83xe2x80x83(10). 
The interval xcex9 of the adjacent ring-shaped zones in the diffractive ring-shaped zone structure formed on the aberration compensating optical element, which is measured in a direction perpendicular to the optical axis, is expressed by the following formula (11):
xcex9=mxc2x7xcex0/(d"PHgr"/dh)xe2x80x83xe2x80x83(11), 
where xcex0 is the optimum wavelength, and m is the diffraction order of the diffracted light having the maximum diffracted light amount.
When the formula (11) is substituted for the formula (10), the interval xcex9 of the adjacent ring-shaped zones in the diffractive ring-shaped zone structure is determined by the following formula (12):
xcex9=mxc2x7xcex0/(xcfx86SAxc2x7h)xe2x80x83xe2x80x83(12). 
Therefore, in the above-described aberration compensating optical element for DVD, when the optimum wavelength for the ring-shaped zone structure is 650 nm, the interval xcex9650 of the diffractive ring-shaped zone structure in the height 2 mm from the optical axis, is expressed by the following formula (13):
xcex9650=mxc2x7650xc3x9710xe2x88x923/(1/92.3xc2x72)=30xc2x7m (xcexcm)xe2x80x83xe2x80x83(13). 
On the other hand, in the above-described aberration compensating optical element for a high density storage optical pickup device, when the optimum wavelength for the ring-shaped zone structure is 405 nm, the interval xcex9405 of the diffractive ring-shaped zone structure in the height 2 mm from the optical axis, is expressed by the following formula (14):
xcex9405=mxc2x7405xc3x9710xe2x88x923/(1/34.1xc2x72)=6.9xc2x7m (xcexcm)xe2x80x83xe2x80x83(14). 
In the formula (14), when the diffractive ring-shaped zone structure is determined so that the first order diffracted light has the maximum diffracted light amount, the interval of the adjacent ring-shaped zones in the diffractive ring-shaped zone structure is 6.9 xcexcm in the position corresponding to the entrance pupil of the objective lens. Therefore, there is some possibility that a light mount loss occurring due to a phase non-matching portion of the ring-shaped zone, which is caused by the transferring the shape of edge portion of a diamond tool, when a mold for injection molding is processed by cutting with SPDT, influences greatly. Further, in case that the interval of the ring-shaped zones in the diffractive ring-shaped zone structure is small, it is difficult that the shape of the ring-shaped zone is transferred in the molding. Thereby, a light amount loss occurring due to a phase non-matching portion also influences greatly.
As described above, like the aberration compensating optical element according to the second aspect of the present invention, the difference xcex94 (mm) in a direction of an optical axis between adjacent steps of the plurality of ring-shaped zone steps of the aberration compensating optical element is determined so that a diffracted light amount of the higher order diffracted light having a diffraction order which is m or more (where m is an integer of not less than 2), is larger than those of the diffracted lights having the other diffraction orders. From the above formula (14), the interval of the adjacent ring-shaped zones in the diffractive ring-shaped zone structure can be m times wider. Therefore, the influence of a light amount loss caused by the phase non-matching portion can be small.
Further, like the aberration compensating optical element according to the third aspect of the present invention, when the diffractive ring-shaped zone structures are formed on n (n is an integer of not less than 2) or more optical surface and the required power xcfx86SA of the aberration compensating optical element is equally divided into n optical surfaces, the power of each surface becomes xcfx86SA/n. From the formula (12), the interval of the ring-shapes zones in the diffractive ring-shaped zone structure can be n times larger. Therefore, the influence of a light amount loss caused by the phase non-matching portion can be small.
For example, the diffractive ring-shaped zone structures are formed on two optical surfaces. Further, the difference xcex94 (mm) in a direction of an optical axis between adjacent steps of the plurality of ring-shaped zone steps of the aberration compensating optical element is determined so that a diffracted light amount of the higher order diffracted light having a diffraction order of 2, is larger than those of the diffracted lights having the other diffraction orders. In this case, from the formulas (12) and (14), the interval of the adjacent ring-shaped zones in the diffractive ring-shaped zone structure can be 27.6 xcexcm in the position corresponding to the entrance pupil of the above objective lens.
In accordance with the fourth aspect of the present invention, an aberration compensating optical element comprises:
a single lens;
wherein the single lens has one optical surface having a diffractive structure having a plurality of ring-shaped zone steps formed on a plane surface and another optical surface opposite to the one optical surface, which has a concave refractive surface; and
wherein the aberration compensating optical element is adapted for being disposed on an optical path between a light source for emitting a light having a wavelength of not more than 550 nm, and an objective lens made of a material having an Abbe constant of not more than 95.0 at a d-line.
In the present invention, the phrase xe2x80x9ca diffractive structure having a plurality of ring-shaped zone steps is formed on a plane surfacexe2x80x9d has the same meaning that a line (an envelope) by which the peaks of the ring-shaped zone steps are connected with each other, becomes a straight line in FIGS. 2A and 12A described below.
When the diffractive ring-shaped zone structure is formed on a plane surface, the reflected light which is reflected by the diffractive structure, travels in a different direction from the incident light. Therefore, it can be prevented that a ghost signal is generated in a detection system of the optical pickup device. Further, because the reflecting power of the optical surface on which the diffractive ring-shaped zone structure is formed, is 0, the total power which is the sum of the refractive power and the diffractive power, can be calculated by the formula (7). A refractive surface having a negative refractive power is formed on an optical surface opposite to the optical surface on which the diffractive structure is formed, so that the absolute value of the refractive power of the refracting surface is same as that of the power calculated by the formula (7). Thereby, the power of the whole system of the aberration compensating optical element can be 0. Therefore, it becomes easy that the aberration compensating optical element is disposed in the parallel light flux.
Further, when the diffractive ring-shaped zone structure is formed on a plane surface, it is possible that the aberration compensating optical element for a high density storage optical pickup device, in which the interval of the ring-shaped zone in the diffraction ring-shaped structure is several micro-meter as calculated by the above formula (14), can be prepared by using the electron beam drawing by which a fine diffractive structure can be formed without the form error. The method for preparing the fine diffractive structure by using the electron beam drawing is disclosed in xe2x80x9cOPTICAL DESIGN, Journal of Optics Design Group, No. 20, 2000.2.25, pp.26-31xe2x80x9d.
In the above-described aberration compensating optical element, the following formula (15) may be satisfied:
0.5xc3x9710xe2x88x922 less than PD less than 15.0xc3x9710xe2x88x922xe2x80x83xe2x80x83(15), 
where PD is a paraxial power (mmxe2x88x921) of the diffractive structure and is defined by the following equation:
PD=xcexa3(xe2x88x922xc2x7b2ixc2x7ni), 
when an optical path difference function is defined by the following equation:
"PHgr"bi=nixc2x7(b2ixc2x7hi2+b4ixc2x7hi4+b6ixc2x7hi6+ . . . ), 
as a function that an optical path difference "PHgr"bi added to a wavefront transmitting through the aberration compensating optical element, by the diffractive structure formed on an i-th surface of the aberration compensating optical element, is expressed by using a height hi (mm) from the optical axis; where ni is a diffraction order of a diffracted light having a maximum diffracted light amount among a plurality of diffracted lights generated by the diffractive structure formed on the i-th surface, and b2i, b4i, b6i, . . . are a second order coefficient of the optical path difference function, a fourth order one, a sixth order one . . . , respectively.
As described above, for example, in order to compensate the axial chromatic aberration occurring at the objective lens having a focal length of 2.35 mm, a working wavelength of 405 mm, an NA of 0.85, an entrance pupil with a diameter xcfx86 of 4 mm, and a xcexdd of 55, the aberration compensating optical element for a high density storage optical pickup device is required to have a diffractive power xcfx86SA of about 2.9xc3x9710xe2x88x922 (mmxe2x88x921). In practice, because an objective lens for a high density storage optical pickup device has a large NA, a focal depth is small. Therefore, it is required that the axial chromatic aberration is precisely compensated. The required power of the aberration compensating optical element for a high density storage optical pickup device becomes larger than the above value. Further, the required power of the aberration compensating optical element is changed according to the focal length of the objective lens and the Abbe constant. Therefore, like the above formula (15), the condition is determined as a preferable range of the power of the aberration compensating optical element for a high density storage optical pickup device.
At the lower limit of the formula (15), the axial chromatic aberration of the wave surface condensed on the information recording surface of the optical information recording medium is not too under-corrected. At the upper limit of the formula (15), the axial chromatic aberration of the wave surface condensed on the information recording surface of the optical information recording medium is not too over-corrected. In order to obtain the above function, it is more preferable that the following formula (16) is satisfied:
1.0xc3x9710xe2x88x922 less than PD less than 10.0xc3x9710xe2x88x922xe2x80x83xe2x80x83(16). 
In the above-described aberration compensating optical element, it is preferable that the paraxial power Pxcex0 of the aberration compensating optical element is substantially zero at the wavelength xcex0 of the light emitted from the light source. Therefore, it becomes easy that the aberration compensating optical element is disposed in the parallel light flux. In the concrete, wherein the following formulas (17) to (19) are satisfied:
PD greater than 0xe2x80x83xe2x80x83(17) 
PR less than 0xe2x80x83xe2x80x83(18) 
xe2x88x920.9 less than PD/PR less than xe2x88x921.1xe2x80x83xe2x80x83(19), 
where PR is a refractive power (mmxe2x88x921) of the aberration compensating optical element as a refractive lens.
In the above-described aberration compensating optical element, it is preferable that the diffractive structure has such a spherical aberration property that a spherical aberration of an emergent light flux is changed in an under-corrected direction or an over-corrected direction when a wavelength of an incident light flux is shifted to a longer wavelength side;
wherein the diffractive structure is formed so as to satisfy the following inequality:
0.2xe2x89xa6|(Phf/Phm)xe2x88x922|xe2x89xa66.0xe2x80x83xe2x80x83(20), 
where Phf is a first interval in a direction to perpendicular to an optical axis of the diffractive structure between adjacent steps of the ring-shaped zones of the diffractive structure at a diameter hf which is a half of a maximum effective diameter hm, and Phm is a second interval in the direction to perpendicular to the optical axis of the diffractive structure between adjacent steps of the ring-shaped zones of the diffractive structure at the maximum effective diameter hm.
In case that a short wavelength light source emitting a light having a wavelength of not more than 550 nm, in particular, about 400 nm, as described above, the refractive index change of the lens material, which is caused by a slight wavelength change, becomes large. Therefore, when the slight wavelength change is caused, the axial chromatic aberration is caused at the objective lens and the spherical aberration caused at the objective lens is changed. For example, in case of an objective lens having a single lens structure, when the wavelength is shifted to 10 nm longer wavelength side than the design wavelength, the spherical aberration is changed in an over-corrected direction. In case of an objective lens having a doublet structure, when the wavelength is shifted to 10 nm longer wavelength side than the design wavelength, the spherical aberration is changed in an over-corrected direction or in an under-corrected direction, according to a power arrangement in the lens group.
The formula (20) is a condition to compensate the spherical aberration change caused at the objective lens, by a diffracting function of the aberration compensating optical element. If the optical path difference function has only second order coefficient of the optical path difference function (also referred to as xe2x80x9ccoefficient of a diffractive surface), the condition is (Phf/Phm)xe2x88x922=0. However, in the aberration compensating optical element according to the present invention, higher order coefficients of the optical path difference function are used in order to precisely compensate the spherical aberration change which is caused at the objective lens by a slight wavelength change from the design wavelength, with the diffracting function of the diffractive structure in the aberration compensating optical element. Therefore, it is preferable that the formula (Phf/Phm)xe2x88x922 has a value apart from 0 to a certain degree. If the formula (20) is satisfied, the above spherical aberration change is excellently cancelled by the diffracting function.
In the above-described aberration compensating optical element, when a wavelength of a light entering the diffractive structure is not more than 550 nm, it is preferable that a diffraction efficiency of the diffractive structure becomes maximal. More preferably, the design wavelength of the objective lens is substantially the same as the wavelength at which the diffraction efficiency becomes maximal.
In the above-described aberration compensating optical element, it is preferable that the aberration compensating optical element is a plastic lens. As an optical plastic material, for example, olefin resin, polymethylmethacrylate, styrene acrylonitrile, polycarbonate, thermosetting plastics, polystyrene or the like is exemplified. Preferable optical plastic material has an internal transmittance of not less than 80% when the transmitted light has the wavelength of not more than 550 nm and the optical plastic has the thickness of 3 mm. In case that the aberration compensating optical element is disposed in an optical pickup device as an element separated from the objective lens like the aberration compensating optical element according to the present invention, it is not strongly required that to thin the aberration compensating optical element or to downsize it like the objective lens in order to obtain the operating distance and to lighten it. Therefore, a refraction-diffraction-integrated type of optical element lens which is made of the optical plastic material, can be easily produced by the injection molding method or the like using a mold at a low cost.
In accordance with the fifth aspect of the present invention, an optical system for carrying out at least one of a record of information on an information recording surface of an optical information recording medium and a reproduction of information from the information recording surface; comprises:
a light source for emitting a light having a wavelength of not more than 550 nm;
an objective lens made of a material having an Abbe constant of not more than 95.0 at a d-line; and
any one of the above-described aberration compensating optical elements, which is disposed on an optical path between the light source and the objective lens.
In accordance with the sixth aspect of the present invention, an optical pickup device for carrying out at least one of a record of information on an information recording surface of an optical information recording medium and a reproduction of information from the information recording surface; comprises:
an optical system comprising: a light source for emitting a light having a wavelength of not more than 550 nm; an objective lens made of a material having an Abbe constant of not more than 95.0 at a d-line; and an aberration compensating optical element, which is disposed on an optical path between the light source and the objective lens;
wherein the optical pickup device comprises the above-described optical system as a condensing optical system.
According to the above-described optical system, it is possible to realize an optical system for optical pickup device and an optical pickup device, which can compensate the axial chromatic aberration with a relatively simple structure even though a light source having a bad monochromaticity or a light source in which a wavelength of the light suddenly changes, is used in a high density storage optical pickup system or the like, and which can be manufactured in a low cost.
In the present invention, the optical information recoding medium includes not only a current optical information recoding medium having a disk-shape, for example, each type of CD, such as CD, CD-R, CD-RW, CD-Video, CD-ROM or the like, and each type of DVD, such as DVD, DVD-ROM, DVD-RAM, DVD-R, DVD-RW, DVD+RW or the like, and MD or the like but also a next generation high density recording medium or the like.
The above-described optical pickup device according to the present invention, can be provided in a recorder and a reproducer for at least one of a sound and an image, for example, a player or a drive which is compatible with an optical information recording medium, such as CD, CD-R, CD-RW, CD-Video, CD-ROM, DVD, DVD-ROM, DVD-RAM, DVD-R, DVD-RW, DVD+RW, MD or the like, or an AV apparatus, a personal computer or other information terminals into which the player or the drive is incorporated, or the like.
In accordance with the seventh aspect of the present invention, an aberration compensating optical element comprises:
a plastic lens having a single lens structure, and comprising a diffractive structure having a plurality of ring-shaped zone steps formed on at least one surface of the plastic lens;
wherein the aberration compensating optical element is adapted for being disposed on an optical path between a light source and an objective lens having an image-side numerical aperture of not less than 0.75 and comprising at least one plastic lens; and
wherein the aberration compensating optical element decreases a change xcex943SAOBJ in a third-order spherical aberration of the objective lens, which is caused by a refractive index change xcex94NOBJ of at least one plastic lens contained in the objective lens due to a temperature change of the objective lens, by using an inclination change of a marginal ray of an emergent light flux from the aberration compensating optical element, which is caused by a refractive index change xcex94NAC of the aberration compensating optical element due to a temperature change of the aberration compensating optical element.
As described above, the aberration compensating optical element according to the present invention, comprises a plastic lens comprising a diffractive structure having a plurality of ring-shaped zone steps formed into substantially concentric circles on at least one surface of the plastic lens; wherein the aberration compensating optical element is adapted for being disposed on an optical path between a light source and an objective lens having an image-side numerical aperture of not less than 0.75 and comprising at least one plastic lens. A general optical plastic material has a property that when the temperature rises, the refractive index thereof decreases and when the temperature falls, the refractive index thereof increases. An amount of the change in the refractive index of the optical plastic material, which is caused by the temperature change, is larger than that of the optical glass material in the number of zeroes. Therefore, the power of the whole system of the aberration compensating optical element which is a plastic lens, is constant. The diffractive structure having a plurality of ring-shaped zone steps is formed into substantially concentric circles on at least one surface of the aberration compensating optical element to suitably distribute the power to the diffractive power as a diffractive lens and the refractive power as a refractive lens. Thereby, it is possible to select an amount of the inclination change of a marginal ray of an emergent light flux from the aberration compensating optical element, which is caused by changing the refractive index change of the aberration compensating optical element due to the temperature change.
As an optical plastic material, for example, olefin resin, polymethylmethacrylate, styrene acrylonitrile, polycarbonate, thermosetting plastics, polystyrene or the like is exemplified. Preferable optical plastic material has an internal transmittance of not less than 80% when the transmitted light has the wavelength of not more than 550 nm and the optical plastic has the thickness of 3 mm.
When the above aberration compensating optical element is disposed on an optical path between the light source and the objective lens comprising at least one plastic lens, it is possible that the change in the third-order spherical aberration of the objective lens, which is caused by a refractive index change of the plastic lens due to the temperature change, is changed so as to cancel it by the inclination change of the marginal ray of the emergent light flux from the aberration compensating optical element. Therefore, even though a high NA objective lens having at least one plastic lens, which has a narrow usable temperature range, is used, the usable temperature range can be expanded by using the objective lens with the aberration compensating optical element according to the present invention. As a result, in a high density storage optical pickup system in which the objective lens having an image-side numerical aperture of not less than 0.75 is required, a plastic lens can be used as an objective lens. Therefore, it is possible to decrease the cost of the optical pickup device.
In the above-described aberration compensating optical element, it is preferable that the following formula (21) is satisfied:
PT1 less than PT0 less than PT2xe2x80x83xe2x80x83(21), 
where PT0 is a paraxial power (mmxe2x88x921) of the aberration compensating optical element at a predetermined temperature T0;
PT1 is a paraxial power (mmxe2x88x921) of the aberration compensating optical element at a temperature T1 which is lower than the predetermined temperature T0; and
PT2 is a paraxial power (mmxe2x88x921) of the aberration compensating optical element at a temperature T2 which is higher than the predetermined temperature T0.
Because the aberration compensating optical element satisfies the above formula (21), when the temperature rises, an inclination of an upper marginal ray of the emergent light flux from the aberration compensating optical element is changed in a direction of decreasing it as compared with a previous temperature change, that is, in a clockwise direction on the basis of the optical axis. This phenomenon has the same effect as a phenomenon that the magnification of the objective lens is changed in a direction of increasing it. Therefore, in the optical pickup optical device, when the temperature rises, the change in the third-order spherical aberration, which is caused by changing the temperature of the objective lens, can be decreased so as to cancel it by using the aberration compensating optical element with the objective lens having a temperature property that the third-order spherical aberration is changed in an under-corrected direction.
It is preferable that the predetermined temperature T0 is 25xc2x0 C. and that the temperature difference between T0 and T1 and the temperature difference between T0 and T2 are 30xc2x0 C.
It is preferable that the objective lens is one having a doublet lens structure in which a first lens having a positive refractive power and a second lens having a positive refractive power are arranged in an order from a side of the objective lens; and at least the first lens is a plastic lens.
As an objective lens having a temperature property that the third-order spherical aberration component thereof is changed in an under-corrected direction when the temperature rises, the above-described objective lens having a doublet lens structure is exemplified. At least the first lens is a plastic lens. In order to decrease a cost and to lighten the optical system, it is preferable that both of the first lens and the second lens are plastic lenses.
In case that the aberration compensating optical element is used with the above-described objective lens having a doublet lens structure, it is preferable that the aberration compensating optical element according to the present invention satisfies the following formulas (22) and (23):
PR less than 0xe2x80x83xe2x80x83(22) 
0 less than xcex94PAC/xcex94TAC less than 1xc3x9710xe2x88x924xe2x80x83xe2x80x83(23), 
where PR is a refractive power (mmxe2x88x921) of the aberration compensating optical element as a refractive lens; and
xcex94PAC is an amount of a change in a paraxial power (mmxe2x88x921) of the aberration compensating optical element, which is caused by the temperature change xcex94TAC (xc2x0 C.) of the aberration compensating optical element.
When the refractive power of the aberration compensating optical element as a refractive lens satisfies the formula (22), a sign of an amount of the change (xcex94PAC/xcex94TAC) in the paraxial power of the aberration compensating optical element, which is caused by the temperature change, is positive. Therefore, when the temperature changes, the change in the third-order spherical aberration of the objective lens having a doublet lens structure is decreased so as to cancel it. On the other hand, when the value of the formula (xcex94PAC/xcex94TAC) is smaller than the upper limit of the formula (22), when the temperature changes, the third-order spherical aberration of the objective lens having a doublet lens structure is not too compensated. As described above, when the value of the formula (xcex94PAC/xcex94TAC) satisfies the formula (22), it is possible to suitably compensate the spherical aberration change of the objective lens having a doublet lens structure, which is caused by the temperature change.
It is preferable that the light source is one for emitting a light having a wavelength of not more than 550 nm; and
wherein the following formula (24) is satisfied:
Pxcex1 less than Pxcex0 less than Pxcex2xe2x80x83xe2x80x83(24), 
where Pxcex0 is a paraxial power (mmxe2x88x921) of the aberration compensating optical element at the wavelength xcex0 of the light emitted from the light source;
Pxcex1 is a paraxial power (mmxe2x88x921) of the aberration compensating optical element at a wavelength xcex1 which is 10 nm shorter than the wavelength xcex0; and
Pxcex2 is a paraxial power (mmxe2x88x921) of the aberration compensating optical element at a wavelength xcex2 which is 10 nm longer than the wavelength xcex0.
As described above, because the aberration compensating optical element has a wavelength property to satisfy the formula (24), when a light having a different wavelength from one having a predetermined wavelength by a predetermined wavelength difference, enters the aberration compensating optical element, the axial chromatic aberration occurring at the aberration compensating optical element and the axial chromatic aberration occurring at the objective lens are cancelled to compensate the axial chromatic aberration caused by the wavelength change. Therefore, the axial chromatic aberration is minimized at the condensing spot formed by condensing the light transmitting the aberration compensating optical element and the objective lens on the information recording surface of the optical information recording medium. By using the aberration compensating optical element according to the present invention with the objective lens, even though the axial chromatic aberration caused at the objective lens is not precisely compensated, the objective lens can be used as an objective lens for a high density optical information recording medium.
When the change in the third-order spherical aberration component of the objective lens, which is caused by the temperature change, is decreased so as to cancel it, in order to minimize the remaining aberration of the whole system, it is preferable that the objective lens which is used with the aberration compensating optical element, satisfies the formula (25). Thereby, when the change in the third-order spherical aberration component of the objective lens, which is caused by the temperature change, is decreased so as to cancel it, it is possible to minimize the remaining aberration.
The following formulas (26) to (28) are conditions for decreasing the change in the spherical aberration of the objective lens, which is caused by the temperature change, so as to excellently cancel it and for minimizing the axial chromatic aberration at the condensing spot on the information recording surface of the optical information recording medium. In case that when the temperature changes, the change in the third-order spherical aberration of the objective lens satisfies the formula (26) and the axial chromatic aberration occurring the objective lens satisfies the formula (27), the diffractive structure having a diffractive power which satisfies the formula (28) is formed on the aberration compensating optical element. Thereby, the compensation for the temperature property of the objective lens and the compensation for the axial chromatic aberration occurring at the objective lens are compatible.
|xcex943SAOBJ|/|xcex945SAOBJ| greater than 1xe2x80x83xe2x80x83(25), 
xe2x88x9230.0xc3x9710xe2x88x924 less than xcex943SAOBJ/(xcex94TOBJxc2x7NA4xc2x7fOBJ) less than 0xe2x80x83xe2x80x83(26), 
3xc3x9710xe2x88x922 less than xcex94fBOBJxc2x7xcexddOBJ/fOBJ less than 14xc3x9710xe2x88x922xe2x80x83xe2x80x83(27), 
1.0xc3x9710xe2x88x922 less than PD less than 10.0xc3x9710xe2x88x922xe2x80x83xe2x80x83(28), 
where xcex943SAOBJ is a change in a third-order spherical aberration component of a Zernike polynomial into which an aberration of the objective lens is expanded, in case that a refractive index of the plastic lens in the objective lens is changed by xcex94NOBJ due to the temperature change xcex94TOBJ (xc2x0 C.) of the objective lens; the change in the third-order spherical aberration being expressed by an RMS (root mean square value) by a wavelength xcex0 of a light emitted from the light source; and a sign of the change in the third-order spherical aberration being positive when the third-order spherical aberration component is changed in an over-corrected direction, and being negative when the third-order spherical aberration component is changed in an under-corrected direction;
xcex945SAOBJ is a change in a fifth-order spherical aberration component of the Zernike polynomial into which the aberration of the objective lens is expanded, in case that the refractive index of the plastic lens in the objective lens is changed by xcex94NOBJ due to the temperature change xcex94TOBJ (xc2x0 C.) of the objective lens; the change in the fifth-order spherical aberration being expressed by an RMS (root mean square value) by the wavelength xcex0 of a light emitted from the light source;
NA is a predetermined image-side numerical aperture which is required for at least one of a record of information on an optical information recording medium and a reproduction of information from the optical information recording medium;
fOBJ is a focal length (mm) of the objective lens;
xcex94fBOBJ is an axial chromatic aberration (mm) occurring at the objective lens when a light having a wavelength which is +10 nm longer than the wavelength xcex0 of a light emitted from the light source enters the objective lens;
xcexddOBJ is a mean value of an Abbe constant of the first lens in the objective lens at the d-line and an Abbe constant of the second lens at the d-line; and
PD is a paraxial power (mmxe2x88x921) of the diffractive structure and is defined by the following equation:
PD=xcexa3(xe2x88x922xc2x7b2ixc2x7ni), 
when an optical path difference function is defined by the following equation:
"PHgr"bi=nixc2x7(b2ixc2x7hi2+b4ixc2x7hi4+b6ixc2x7hi6+ . . . ), 
as a function that an optical path difference "PHgr"bi added to a wavefront transmitting through the diffractive structure formed on an i-th surface of the aberration compensating optical element, by the diffractive structure formed on the i-th surface, is expressed by using a height hi (mm) from an optical axis; where b2i, b4i, b6i, . . . are a second order coefficient of the optical path difference function, a fourth order one, a sixth order one . . . , respectively, and ni is a diffraction order of a diffracted light having a maximum diffracted light amount among a plurality of diffracted lights generated by the diffractive structure formed on the i-th surface.
It is preferable that the above-described aberration compensating optical element comprises one optical surface on which the diffractive structure having a plurality of macroscopically plane ring-shaped zone steps is formed, that is, one optical surface having a diffractive structure having a plurality of ring-shaped zone steps formed on a plane surface and another optical surface opposite to the one optical surface, which has a concave refractive surface.
When the diffractive surface having the diffractive ring-shaped zone structure is formed on a plane surface, the reflected light which is reflected by the diffractive structure, travels in a different direction from the incident light. Therefore, it can be prevented that a ghost signal is detected by a detection system of the optical pickup device. Further, the diffractive structure can be precisely formed by the electron beam drawing. The method for preparing the fine diffractive structure having fine ring-shaped zones by using the electron beam drawing is disclosed in xe2x80x9cOPTICAL DESIGN, Journal of Optics Design Group, No. 20, 2000.2.25, pp.26-31xe2x80x9d. The optical surface opposite to the plane surface on which the diffractive structure is formed, is a concave refractive surface. When the refractive power of the concave surface has an absolute value which is substantially the same as that of the diffractive power of the diffractive structure and the sign of the refractive power is opposite to that of the diffractive power, the power of the whole system of the aberration compensating optical element can be 0. Therefore, it becomes easy that the aberration compensating optical element is disposed in the parallel light flux.
In the above-described aberration compensating optical element, it is preferable that the paraxial power Pxcex0 of the aberration compensating optical element is substantially zero at the wavelength xcex0 of the light emitted from the light source. Therefore, it becomes easy that the aberration compensating optical element is disposed in the parallel light flux. In the concrete, wherein the following formulas (29) to (31) are satisfied:
PD greater than 0xe2x80x83xe2x80x83(29) 
PR less than 0xe2x80x83xe2x80x83(30) 
xe2x88x920.9 less than PD/PR less than xe2x88x921.1xe2x80x83xe2x80x83(31). 
It is preferable that the above-described aberration compensating optical element has a temperature property that when the temperature rises, the paraxial power is changed in a direction of decreasing it, and the following formula (32) is satisfied:
PT2 less than PT0 less than PT1xe2x80x83xe2x80x83(32), 
where PT0 is a paraxial power (mmxe2x88x921) of the aberration compensating optical element at a predetermined temperature T0;
PT1 is a paraxial power (mmxe2x88x921) of the aberration compensating optical element at a temperature T1 which is lower than the predetermined temperature T0; and
PT2 is a paraxial power (mmxe2x88x921) of the aberration compensating optical element at a temperature T2 which is higher than the predetermined temperature T0.
Because the aberration compensating optical element satisfies the above formula (32), when the temperature rises, an inclination of an upper marginal ray of the emergent light flux from the aberration compensating optical element is changed in a direction of increasing it as compared with a previous temperature change, that is, in a counterclockwise direction on the basis of the optical axis. This phenomenon has the same effect as a phenomenon that the magnification of the objective lens is changed in a direction of decreasing it. Therefore, in the optical pickup optical device, when the temperature rises, the change in the third-order spherical aberration, which is caused by changing the temperature of the objective lens, can be decreased so as to cancel it by using the aberration compensating optical element with the objective lens having a temperature property that the third-order spherical aberration is changed in an over-corrected direction.
It is preferable that the predetermined temperature T0 is 25xc2x0 C. and that the temperature difference between T0 and T1 and the temperature difference between T0 and T2 are 30xc2x0 C.
As an objective lens having a temperature property that the third-order spherical aberration component is changed in an over-corrected direction when the temperature rises, a plastic lens having a single lens structure is exemplified. When the numerical aperture of the plastic lens having a single lens structure increases, the usable temperature range thereof becomes very narrow (the change in the spherical aberration of the plastic lens having a single lens structure, which is caused by the temperature change, is about 5 to 10 times larger than that of the plastic lens having a doublet lens structure, which has the same focal length, image-side numerical aperture, working wavelength and the magnification.) Therefore, a temperature controlling device for controlling the temperature of the plastic lens is specially required. As a result, the increase in the manufacturing cost of the optical pickup device and the complication of the optical pickup device are caused. By using the aberration compensating optical element according to the present invention with a high NA plastic lens having a single lens structure, the usable temperature range of a high NA plastic lens having a single lens structure can be expanded with a simple structure and at a low cost.
It is preferable that at least one ring-shaped zone step having a step distance xcex94 (mm) in a direction of an optical axis between adjacent steps of the plurality of ring-shaped zone steps is formed within an effective diameter so that m, defined by following equations:
m=INT(Y), 
Y=xcex94xc3x97(nxe2x88x921)/(xcex0xc3x9710xe2x88x923)xe2x80x83xe2x80x83(33), 
is an integer except 0 and xc2x11,
where INT(Y) is an integer obtained by rounding Y, xcex0 is the wavelength (nm) of the light emitted from the light source, and n is a refractive index of the aberration compensating optical element at the wavelength xcex0 (nm).
The above formula (33) has a meaning that the difference xcex94 (mm) in a direction of an optical axis between adjacent steps of the plurality of ring-shaped zone steps of the aberration compensating optical element is determined so that a diffracted light amount of the diffracted light having a diffraction order which is two or more, is larger than those of the diffracted lights having the other diffraction orders, when the incident light flux is diffracted by the diffractive structure formed on the optical surface of the aberration compensating optical element. Thereby, as compared with the case that the difference xcex94 is determined so that the first order diffracted light has a maximum diffracted light amount, the minimum value of the interval of the adjacent ring-shaped zone steps is m times larger. Therefore, a light amount loss occurring due to the phase non-matching portion caused by a transfer defect of the ring-shaped zone steps in the molding, is decreased. Because the number of the ring-shaped zone steps is decreased by 1/m, it is possible to shorten the processing time of a die for the molding.
It is preferable that the above-described aberration compensating optical element comprises two diffractive structures having a plurality of ring-shaped zone steps formed on both surfaces. By distributing the power of the diffractive structure to two optical surface, a minimum value of the interval of the adjacent ring-shaped zone steps in the optical axis direction, is larger as compared with the case that the diffractive structure is formed on only one surface. Therefore, a light amount loss occurring due to the phase non-matching portion caused by a transfer defect of the ring-shaped zone steps in the molding, is decreased.
In the above-described aberration compensating optical element, it is preferable that the diffractive structure has such a spherical aberration property that a spherical aberration of an emergent light flux is changed in an under-corrected direction or an over-corrected direction when a wavelength of an incident light flux is shifted to a longer wavelength side; and
when the diffractive structure is formed so as to satisfy the following inequality:
0.2xe2x89xa6|(Phf/Phm)xe2x88x922|xe2x89xa66.0xe2x80x83xe2x80x83(34), 
where Phf is a first interval in a direction to perpendicular to an optical axis of the diffractive structure between adjacent steps of the ring-shaped zones of the diffractive structure at a diameter hf which is a half of a maximum effective diameter hm, and Phm is a second interval in the direction to perpendicular to the optical axis of the diffractive structure between adjacent steps of the ring-shaped zones of the diffractive structure at the maximum effective diameter hm.
In case that a short wavelength light source emitting a light having a wavelength of not more than 550 nm, in particular, about 400 nm, the refractive index change of the lens material, which is caused by a slight wavelength change, becomes large. Therefore, when the slight wavelength change is caused, the axial chromatic aberration is caused at the objective lens and the spherical aberration caused at the objective lens is changed. For example, in case of an objective lens having a single lens structure, when the wavelength is shifted to 10 nm longer wavelength side than the design wavelength, the spherical aberration is changed in an over-corrected direction. In case of an objective lens having a doublet structure, when the wavelength is shifted to 10 nm longer wavelength side than the design wavelength, the spherical aberration is changed in an over-corrected direction or in an under-corrected direction, according to a power arrangement in the lens group.
The formula (34) is a condition to compensate the spherical aberration change caused at the objective lens, by a diffracting function of the aberration compensating optical element. If the optical path difference function has only second order coefficient of the optical path difference function (also referred to as xe2x80x9ccoefficient of a diffractive surface), the condition is (Phf/Phm)xe2x88x922=0. However, in the aberration compensating optical element according to the present invention, higher order coefficients of the optical path difference function are used in order to precisely compensate the spherical aberration change which is caused at the objective lens by a slight wavelength change from the design wavelength, with the diffracting function of the diffractive structure in the aberration compensating optical element. Therefore, it is preferable that the formula (Phf/Phm)xe2x88x922 has a value apart from 0 to a certain degree. If the formula (34) is satisfied, the above spherical aberration change is decreased so as to excellently cancel it by the diffracting function.
In the above-described aberration compensating optical element, when a wavelength of a light entering the diffractive structure is not more than 550 nm, it is preferable that a diffraction efficiency of the diffractive structure becomes maximal. More preferably, the design wavelength of the objective lens is substantially the same as the wavelength at which the diffraction efficiency becomes maximal.
In accordance with the eighth aspect of the present invention, an optical system for carrying out at least one of a record of information on an information recording surface of an optical information recording medium and a reproduction of information from the information recording surface; comprises:
a light source;
an objective lens having an image-side numerical aperture of not less than 0.75 and comprising at least one plastic lens; and
any one of the above-described aberration compensating optical elements, which is disposed on an optical path between the light source and the objective lens.
In accordance with the ninth aspect of the present invention, an optical pickup device for carrying out at least one of a record of information on an information recording surface of an optical information recording medium and a reproduction of information from the information recording surface; comprises:
an optical system comprising: a light source; an objective lens having an image-side numerical aperture of not less than 0.75 and comprising at least one plastic lens; an aberration compensating optical element, which is disposed on an optical path between the light source and the objective lens;
wherein the optical pickup device comprises the above-described optical system as a condensing optical system.
According to the above-described optical system and the optical pickup device, even though a high NA plastic objective lens is used in a high density storage optical pickup system or the like, the change in the spherical aberration occurring at the objective lens, which is caused by the temperature change, can be minimized with a relatively simple structure.
In accordance with the tenth aspect of the present invention, in a recorder and a reproducer, the above-described optical pickup device is provided. It is possible to carry out the record of at least one of a sound and an image and the reproduction of at least one of a sound and an image, respectively.
In this specification, the diffractive surface is a surface having a function of diffracting the incident light flux by providing a relief on a surface of the optical element, for example, a surface of a lens. In case of one surface having a region that the diffraction occurs and a region that the diffraction does not occur, the diffractive surface is a region that the diffraction occurs. The diffractive structure or the diffraction pattern is the region that the diffraction occurs. As a shape of the relief, for example, a plurality of ring-shaped zones which are formed into substantially concentric circles around the optical axis on the surface of the optical element, are known. Further, the plurality of ring-shaped zones have a sectional form in which each ring-shaped zone step has a shape of saw teeth or a shape of steps on the cross section including the optical axis. The shape of the relief includes the above-described shape.
In this specification, the record of information and the reproduction of information are that the information is recorded on the information recording surface of the optical information recording medium and that the information recorded on the information recording surface is reproduced, respectively. The condensing optical system according to the present invention, may be used to carry out the record only or the reproduction only, or to carry out both the record and the reproduction. The condensing optical system may carry out the record for one optical information recording medium and carry out the reproduction for another optical information recording medium. Further, the condensing optical system may carry out the record or the reproduction for one optical information recording medium and carry out both the record and the reproduction for another optical information recording medium. In the specification, the reproduction includes that the information is only read out.